WiGig standards that are wireless communication standards using a millimeter wave band signal employ a modulated signal having a broad band channel width on the order of 1.76 GHz. Further, a modulation scheme adopted in the WiGig standards is a TDD (Time Division Duplex) scheme. Since the minimum duration of a transmission slot is a few microseconds, a modulated signal detection circuit capable of detecting a modulated signal with high accuracy and high resolution even when a transmission operating time is short is of importance.
In order to improve resolution of a digital signal output from the modulated signal detection circuit, what is needed is to use an A/D converter (ADC: A/D converter) having a large number of bits. However, since circuitry of the A/D converter having a large number of bits is complicate, a circuit size and power consumption increase. Against the backdrop, a method for enhancing resolution of a digital signal without changing the number of bits of the A/D converter has long been desired.
FIG. 14 is a block diagram of a related art A/D conversion circuit described in connection with Patent Literature 1. The related art A/D conversion circuit shown in FIG. 14 has a level detection circuit 4 that detects a level of an input signal; reference power sources 3a, 3b, and 3c that output reference voltages having a plurality of different voltage values in a switchable manner by means of an output switch in accordance with a control signal from the level detection circuit 4; and an A/D converter 2 that compares the reference voltages output from the reference power source with an input signal, producing digital outputs.
In the related art A/D conversion circuit, a plurality of comparators that make up the A/D converter 2 compare the plurality of reference voltages output from the reference power sources with an input signal, converting comparison results into digital outputs. When a voltage interval of each of the reference voltages is switched as shown in FIG. 15, the minimum resolution voltage of the A/D converter 2 varies.
The related art A/D conversion circuit detects an amplitude level of the input signal by means of the level detection circuit 4 and switches the reference voltage to be input to the A/D converter 2 to a reference voltage source whose reference voltage has a smaller voltage interval.
As a result of the reference voltage being switched to the reference voltage source, control can be performed in such a way that the input signal closely approximates to the maximum number of bits of the A/D converter 2 and that the A/D converter 2 does not become saturated as the input signal approximates to zero. Accordingly, the related art A/D conversion circuit can enhance the minimum resolution of the A/D converter 2.
However, when detecting a signal of high frequency component which makes up a modulated signal, the related art A/D conversion circuit generates a DC voltage value. On the contrary, when detecting a signal of low frequency component which makes up a modulated signal, the A/D conversion circuit generates an AC voltage value. The modulated signal detection circuit outputs an additional value consisting of the DC voltage value and the AC voltage value.
In the related art A/D conversion circuit, when a level of a modulated signal input to the modulated signal detection circuit becomes higher, a DC value of an average output voltage of the modulated signal detection circuit varies to a greater voltage value in accordance with a detected voltage level of the modulated signal.
As above, as a result of the related art A/D converter being used for A/D conversion of a signal output from the modulated signal detection circuit, the minimum resolution of the A/D converter becomes greater as the voltage level of the modulated signal becomes higher. Namely, there has been a problem of difficulty being encountered in enhancing the minimum resolution of the modulated signal detection circuit over an entire input signal range of the A/D converter.
FIG. 16 is a block diagram of a device in which the minimum resolution of an A/D converter is enhanced over an entire input signal range.
The device shown in FIG. 16 includes a superimposed signal generation circuit that generates a high frequency signal whose frequency is higher than that of an input signal; a reference voltage circuit that generates a criterion reference voltage for an A/D converter (ADC), an addition circuit that adds the reference voltage to the high frequency signal, the A/D converter that compares a reference signal, which is an output of the addition circuit, with an input signal, outputting a digital value; and an averaging filter that eliminates the high frequency signal superimposed on the digital value output from the A/D converter.
The reference signal input to the A/D converter is a signal on which there is superimposed a high frequency signal whose frequency is sufficiently higher than that of the input signal; for instance, a high frequency signal whose frequency component is ten times as high as a frequency of the input signal.
FIG. 17 is a graph showing changes in a relationship between a voltage level of a reference signal and a voltage level of an input signal in the A/D converter shown in FIG. 16. In general, a situation in which an operating frequency of the A/D converter is higher than the frequency of the input signal is called “oversampling.”
As mentioned above, the A/D converter compares the input signal on which the high frequency signal is superimposed with a reference signal, outputting a digital value. As shown in FIG. 17, an increase occurs in the number of times the input signal is compared with the reference signal.
A digital value output from the A/D converter is averaged by means of an averaging filter at a predetermined time interval, whereby the averaging filter acquires an output whose bits are larger in number than bits of a digital value output from the A/D converter. Thus, the minimum resolution of the A/D converter can be enhanced over the entire input signal range.